Registered sale deed has been executed in favour of your mother in law  and wife 

2)once sale deed has been executed they are absolute owners of property 

3) builder has no right ,title interest on said flat 

4) they dont need builder consent to sell one third share in flat to you 

Letter from advocate is not necessary 

Regarding payment of stamp duty approach sub registrar office for necessary clarifications 

A supplemental agreement can be very much be executed and registered as you name as buyer. There is no legal bar against registration of such agreement. Adding a party as claimant does not entail any additional stamp duty. Issue a common lawyer's notice by buyers  to seller for joining registration add you as buyer furnishing draft agreement warning of legal action. 

As of now you can invest in the NABARD bond's those amount to save tax and later on after getting possession of property in your wife's hand than you can purchase partly share in that property.

Or ask builder to perform rectification deed as he forgot to add your name in the property. But the sale deed should not registered more than a month.

If the builder has already executed a sale deed and registered the same on the names of the purchasers i.e. your wife and mother in law, then your name cannot be included in the sale deed.

However if it is still a registered sale agreement only and not a sale deed, then the builder may requested to cancel the sale agreement  and execute a fresh one in the names of all the three so that in future he can execute the sale deed in all the three names.

Alternately you can request the builder to execute a supplementary sale agreement by including your name with the consent of the existing purchasers for which also you may have to pay the stamp duty and the registration charges as applicable.

If you refer this as sale deed, then the owners are not restricted by the builder to sale a portion of the property by executing a registered sale deed, therefore you can clarify what exactly you are looking for.

A letter from  an advocate will in no way going to come to your rescue to get the desired objects done.

If the existing co purchasers have no objection against inclusion of your name as a co purchaser then I don't see any reason why the builder would have a problem 

The existing co purchasers would not be selling any part of their share to you 

You are just being included as a co purchaser with them 

For that a supplementary agreement has to be made in which it would be stated that all the terms of the principal agreement would remain same and be binding on all the 3 co purchasers now 

So the builder will be the seller, the existing co purchasers would be the confirming parties and you would be the 3rd joint purchaser 

The allotment of the new flat would happen jointly to all the 3 co purchasers under the supplementary agreement 

This agreement would not attract any stamp duty since the duty is already paid on the principal agreement and there would be nominal registration fee 

The builder is concerned with his money. So whether that is paid by 2 or 3 co purchasers it should hardly matter to him 

So whatever contribution you would make towards the sale price to the builder, for that you can claim capital gains tax exemption 

If you go for an assignment of rights by the existing co purchasers to you by paying them certain amount then such assignment would be subject to the main agreement. The builder may ask for a transfer fee in that case. 

Under the Gift deed option as suggested by the builder I fail to understand how he would then bind you under the terms of the main agreement which would contain many protection clauses in favour of the builder 

The gift deed will be between the existing co purchasers and yourself. So how will there be a privity of contract between you and the builder? In case of any breach of the agreement terms the builder cannot sue you as there would be no privity of contract with you. He can only sue his original co purchasers. So his suggestion of gift deed is strange 

Also since no consideration would pass under the gift deed you cannot also claim any capital gains tax exemption 

Yes it will help 

- If the sale deed already registered from the office of the registrar in the name of your wife and her mother , then new name cannot be added, as they become the legal owner of the property .

- Further , if only an agreement is registered , then the builder should not deny to add your name at the time of execution of sale deed in three names . 

- Further, the builder can also enter into a separate agreement with all the owners of the property /purchasers , before registered the sale deed . 

- If not agree then you can approach the consumer forum against him . 

Dear client I am sorry to hear that but in this case you can approach the register amd file an application for the same.